Add Knative metrics to Prometheus

What does this MR do?

Allow for the installation of Knative specific supporting metrics to a pre-existing Prometheus installation when Prometheus is installed via GitLab's Helm Installer. The hooks being introduced are specifically for Knative and the underlying Istio installation to support future work of collecting Knative and Istio analytics data.

Note that no specific data will be collected as a part of this MR.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/54800

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer